The present invention relates to developer resins of the type which are useful in recording materials such as carbonless paper where they function as electron acceptors and react with substantially colorless electron donating compounds to produce a visible image.
Recording materials utilizing developer resins to produce colored images from colorless or substantially colorless materials are well-known. Specific examples of such recording materials include pressure-sensitive carbonless copying paper, heat-sensitive recording paper, electrothermographic recording paper, and the like. They are described in more detail in U.S. Pat. Nos. 2,712,507; 2,730,456; 2,730,457; 3,418,250; 3,432,327; 3,981,821; 3,993,831; 3,996,156; 3,996,405 and 4,000,087, etc. A photographic material has been developed which utilizes this method for forming colored images. See, for example, U.S. Pat. Nos. 4,399,209 and 4,440,846 to The Mead Corporation.
Much research has been directed to developing new and improved developers for use in the aforementioned recording materials. The preferred developers are principally phenol derivatives and phenolic resins. Phenols, biphenols, methylene bis-diphenols, phenol-formaldehyde novolak resins, metal processed novolak resins, salicylic acid derivatives and salts are representative examples of the phenolic developers that have been used. See U.S. Pat. No. 3,934,070 to Kimura teaching salicyclic acid derivatives; U.S. Pat. No. 3,244,550 to Farnham teaching biphenols, diphenols, and resinous products containing them, and U.S. Pat. No. 3,244,549 to Farnham teaching phenol derivatives. Representative examples of phenol-formaldehyde condensates previously used in the art are described in numerous patents, including U.S. Pat. No. 3,672,935.
Among the color developers, phenol-formaldehyde condensates have been widely used because they exhibit excellent color development, good coating properties (rheology) and good water resistance. While phenol-formaldehyde condensates are advantageous color developers, certain questions have arisen regarding their use in recording materials. Because they are prepared from formaldehyde, there is concern that they may be unsafe from both the standpoint of their manufacture and their use recording materials.